In a four-stroke engine, the piston reaches its apex during the compression and exhaust strokes. The distance from the cylinder head to the piston crown during compression determines the compression ratio and is critical to the operation of the invention. This minimum compression space occurs also in the exhaust stroke of the invention and creates an undesirable effect. It would be advantageous to completely clear the cylinder of all exhaust gases. However, only a portion of the gases can be driven off as the piston crown never reaches the valve head assembly.
The present invention solves this problem by permitting the piston to expand during the exhaust stroke and compress during the compression stroke. The result is that, while compression is unaffected, a substantially greater portion of the exhaust gases is driven out of the cylinder during the exhaust stroke.